The present invention relates to a bandwidth reduction system for television signals and it relates especially, though not exclusively, to such a system for use with high definition television (HDTV) signals.
It is accepted that HDTV signals require bandwidth reduction in order to permit them to be broadcast or otherwise disseminate within transmission channels available, and R. Storey has described, in a BBC Research Report PH-280 (BBC RD 1986/5) certain expedients that might be adopted in order to achieve bandwidth reduction of HDTV signals. In particular, Storey contemplates the use of an adaptive filtering system which treats stationary and moving areas of the picture differential, i.e. by imposing different degrees of spatial filtering upon signals deriving from the different areas of the picture, depending upon whether or not a predetermined amount of motion is taking place in each of those areas.
Storey also envisages that the protocol governing the switching at the receiver necessary to accommodate the receiver to the variations in the television signals, caused by the variable filtering, should be controlled by a digital signal, generated at the transmitter, that is determined by the "clean" signals prior to transmission and thus rugged and reliable. The use of this digital control signal is called Digitally Assisted Television (DATV) by Storey.
The spatial filtering adopted by the BBC is shown in FIG. 1 and it can be seen that "diagonal" filtering is used both for static mode and motion mode filtering; the original TV source characteristic being shown as dashed lines.
It has been proposed that step function filtering (i.e with the filtered signal having characteristics similar to, but scaled down from, those of the original TV picture and thus consisting of lines parallel to the two dashed lines in FIG. 1 would be more appropriate for HDTV, but it is believed that the published data has not revealed the optimum filtering arrangements, and neither have the compatible switching protocols been devised.